Cuphead is the perfect introduction to the shoot-em-up genre. The game is difficult but has some of the best tutorialization within the boss fights that makes for a smooth difficulty curve - with a few deliberate outliers. As long as you are open to having a good time with the game and learn from what each boss fight is trying to teach you (where to have your eyes/periphery focused, spatial positioning, etc.), Cuphead delivers a very rewarding experience that will give you the tools to not only master its own game, but you will be well-equipped to dive head first into the shoot-em-up/run-n-gun genre as a whole.

I do think there are some tools the game gives you that allow you to bypass what the game is trying to teach you entirely. This can make for some frustration and a disjointed difficulty curve if you were relying more on these tools in the early game. I believe that each boss was designed with the basic gun and movement options in mind (at least in the first two worlds). I would personally recommend sticking with these basic tools until you are feeling more confident with the game's mechanics. Then you can get weird with it.

The single-player mode is a very catered and balanced experience. Co-op is more difficult but somehow even more fun with the added chaos.